AlGaN/GaN heterojunction field effect transistors (HFETs) are now well developed for delivering high output powers in the 2-8 GHz range. The focus of the research effort has now shifted to increasing their operating frequencies. The most efficient and direct way of increasing the operational frequencies is to reduce the gate length (LG). However, reducing LG to values where the ratio of LG to the AlGaN barrier thickness is below 20-30, normally results in short-channel effects such as the threshold voltage shift and low breakdown voltages. This consequence results from the increased sub-threshold drain-source leakage currents. Apart from the gate to channel separation, the short channel effects and the loss of gate modulation in small gate devices can also result from the poor confinement of the electrons in the two dimensional electron gas (2DEG) channel. For AlGaN/GaN HFETs, two practical approaches that have been reported to achieve better confinement are (1) the use of a double heterostructure (DH) design where the electrons are confined in a thin InGaN channel layer sandwiched between the AlGaN barrier and the GaN buffer layers of the heterojunction and (2) the use of a thin InGaN back barrier layer. It is now well established that gate leakage currents lead to a power degradation in GaN—AlGaN HFETs.
Thus, a need exists for a reduction in the gate leakage currents for many applications where group III nitride HFETs are used.